Product drying apparatus and methods

ABSTRACT

The present invention provides improved apparatus and methods for the monitoring and control of apparatus designed to remove moisture from an initially wet product, such as a continuous dryer ( 14 ). The net rate of water removal from the wet product ( 16 ) is determined during drying thereof, preferably on a real-time basis. A control assembly ( 20 ) is operatively coupled with the dryer ( 14 ) and includes sensors ( 24, 26, 28, 34 ), which are operatively coupled with a digital controller ( 38 ). The controller ( 38 ) has a PID controller operable to continuously determine the average net rate of water removal from the product ( 16 ).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication Ser. 62/437,124 filed Dec. 21, 2016, entitled METHOD OFCONTROLLING PRODUCT DRYING APPARATUS TO PROVIDE THE NET RATE OF WATERREMOVAL FROM A PRODUCT IN REAL TIME. The provisional application isincorporated herein in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention is broadly concerned with methods and apparatusfor the operational monitoring and control of apparatus capable ofremoving water from products passing therethrough, such as dryers orcoolers of the type commonly used in food or feed manufacture. Moreparticularly, the invention provides methods and apparatus permittingdetermination of the net rate of water removal from products duringoperation of said apparatus, preferably on a real time basis.

Description of the Prior Art

During the production of certain comestible products, such as animalfeeds or human foods, an initially dry formula typically containingprotein, starch, and fats is first processed using an extruder or othercooking device to create a continuous stream of cooked product. Theoutput from the extruder is normally too wet for packaging or storage(e.g., from about 20-40% by weight moisture), and thus must be dried. Adryer is positioned to receive the continuous stream of cooked, wetproduct, and to dry the product to a desired moisture level, such as8-12% by weight moisture.

A variety of dryers have been used in the past in these contexts, suchas single or multiple pass horizontal dryers, or vertical dryers.Horizontal dryers of this type include a dryer housing with one or moreinternal conveyors leading from a wet product inlet to a dried productoutlet. Similarly, vertical dryers have a series of stacked decks whereproduct is initially processed in the uppermost deck and is then passedin serial order to the lower decks, leaving to a dried product outlet.In either case, ambient air is drawn into the dryer body and heated,either directly or indirectly, and is then circulated for contact withthe product within the dryer body. In many instances, a cooler sectionis used with product dryers, in order to cool the product for downstreamhandling or packaging; such coolers do not utilize heated air, butmerely circulate air through the dried product to lower the temperaturethereof.

A longstanding problem with such equipment is that it has been necessaryto periodically take samples of the dryer output and physically measurethe moisture content thereof. Only after such testing could theoperation of the equipment be modified in an effort to produceacceptably dried products. Thus, in the context of dryers, duringinitial start-up of the dryers, or in the event of dryer upset, 20minutes or more may elapse before an initial moisture reading can betaken and analyzed in a laboratory. Only then can the dryer operation bemodified, which then entails a further wait until another sample can betaken and measured for moisture content. As a consequence, aconsiderable amount of waste product is generated until it is determinedthat the dryer is operating as required to produce dry product withinspecifications. Thus, the conventional practice of repeated sampling andtesting is inefficient in terms of time and costly in terms of wasteproduct, which has little value or utility.

There is therefore a need in the art for improved dryers and otherapparatus for water removal which can be controlled in such a way as tominimize or eliminate periodic sampling and laboratory moisture testing.

SUMMARY OF THE INVENTION

The present invention overcomes the problems outlined above, andprovides methods for monitoring and controlling the operation ofapparatus serving to remove water from a product via contact between theproduct and input air entering the apparatus, where such apparatus isalso provided with an exhaust fan for moving moisture-laden exit airfrom the apparatus. The methods generally involve the determination ofthe net rate of water removal from the product during operation of theapparatus, preferably in real time.

In one aspect of the invention, such water removal determinationsinvolve the initial determination of the volumetric flow rate of the airfrom the exhaust fan (usually measured as CFM (f³/min) or m³/min), andusing this flow rate to determine the net rate of water removal from theproduct during operation of the apparatus. To this end, determinationsare also made of the net rates of water entering the apparatus from boththe initially wet product to be dried and the input air, and the rate ofwater removed from the apparatus as a part of the exit air.

A number of different techniques may be employed to determine thevolumetric flow rate of the exit air from the fan. Most commonly thesemethods attempt to find air velocity and thereby volumetric flow rate.For example, measurements of air velocity (directly or indirectly), airvelocity pressure and/or static air pressure induced by the fan may bemade. These methods make use of pitot tubes, venturi, orifice plates,vortex shedders, hot wire anemometer, or vane anemometer placed withinthe fan ducting. However, the accuracy of such measurements may becompromised by the fact that the velocity of the exit air is not uniformthroughout the cross-section of a duct, i.e., friction slows the airmoving close to the duct walls, so that the velocity is greater in thecenter of the duct. In light of these considerations, the practice ofthe present invention preferably makes use of indirect methods wherebyone or more operational parameters of the exhaust fan are sensed orotherwise determined, and these parameter(s) may then be used tocalculate volumetric flow rate, such as through the well-known Fan Lawequations. Such fan parameters include the rotational speed (rpm) of thefan, the motor power of the fan (i.e., the energy consumed by the fanmotor to rotate the fan), fan pressure (either static, velocity, ortotal), or combinations thereof.

As noted, the methods of the invention determine and make available thenet rate (usually the average net rate) of water removal from wetproduct passing through the apparatus, and preferably this net rate isdetermined in real time. As used herein, “real time” refers to the factthat the net rate of water removal is determined and available (e.g.,through a visual display) during the time that the initially wet productis passing through the apparatus. Thus, as an amount of initially wetproduct is introduced into the apparatus and passes therethrough, thenet rate of water removal from the amount of initially wet product isdetermined during the time of such passage.

Apparatus in accordance with the invention include all types ofequipment designed to remove water from an initially wet product bycontacting the product with input air to create dried product andmoisture-laden exit air. For example, the apparatus may be in the formof a horizontal or vertical hot-air convection dryers, coolers, or anyother suitable moisture removal equipment.

In preferred practice, the methods of the invention also include thestep of adjusting at least one control parameter of the apparatus whichwill alter the rate of water removal from the product, in response tothe determined net rate of water removal. In one implementation of theinvention, apparatus control involves determining a set point rate SPwhich is the desired net rate of water removal from the product duringpassage thereof through the apparatus, determining a process variable PVwhich is the actual net rate of water exiting the product during passagethrough the apparatus, and determining a control variable CV. CV is anapparatus parameter serving to alter the rate of water removal from theproduct, such as the temperature of heated input air in the case of adryer, the speed of product passing through the apparatus, the contacttime between the product and the input air, and combinations thereof.Apparatus control is achieved by changing the CV as necessary to causethe PV to approach the SP, and ultimately to substantially equal the SP(e.g., within plus or minus 3%, preferably plus or minus 1%, of the SP).Typically, the SP, PV, and CV are successively and periodicallydetermined, and the apparatus operation is controlled using suchsuccessive determinations.

Successive SP, PV, and CV determinations are usually carried out using aPID (proportional-integral-derivative) controller. For example, the PIDcontroller may be operated so that, in each control loop, SP and PV arecalculated, along with the difference between SP and PV; the controlvariable CV is then determined for driving the PV toward the SP. SP maybe determined as the initial water rate (IWR) minus the final water rate(FWR), where IWR is the rate of water delivered to the apparatus as apart of the wet product input, and FWR is the desired rate of waterexiting from the apparatus as a part of the dried product output. PV maybe determined as the rate of water leaving the apparatus as a part ofthe output of the exhaust fan minus the rate of water entering theapparatus as a part of the input air.

In one embodiment, invention provides improved apparatus comprising adrying chamber having a wet product input and a dried product output,one or more input(s) for ambient air, and an output for moisture-ladenexit air including a motor-powered exhaust fan. A sensor assembly isprovided for determining the rotational speed of the exhaust fan and thepower of the exhaust fan motor. The drying chamber also has apparatusfor determining the wet bulb temperatures and dry bulb temperatures ofthe moisture-laden exit air and the input (usually ambient) air. Adigital controller, such as the described PLC/PID controller, isoperably coupled with the sensor assembly and the apparatus in order tocontrol the operation of the apparatus.

As noted above, a variety of different dryers and/or coolers can becontrolled using the invention. For example, the dryer may employdifferent devices for heating ambient-derived input air, such as anopen-flame heater or firebox, or steam coils for indirect heating of theair. Recirculation fans are normally provided for circulating the heatedair between the heating device and the product being dried. Moreover,the exhaust fans of the invention may be of any suitable type, such asconventional rotary blade fans or blowers.

The improved drying methods and apparatus of the invention may form apart of an overall system for the production of products, such as foodor feed products containing amounts of protein (grain- or animal-derivedor both), starch, fats, vitamins, minerals, and other additives. Theseproducts are typically formulated as raw mixtures and are processed tocook the mixtures in order to denature the protein and gelatinize thestarch. Pet feeds, fish feeds, and certain human foods are of thischaracter. Systems of this type include an upstream processing assemblyand a downstream dryer. The upstream components may be conventionalsingle or twin screw extruders, or pellet mills, which feed a continuousstream of wet product to the downstream dryer, which is a dryer inaccordance with the convention. In other contexts, the invention can beused for the control of dryers for fruits, vegetables, nuts, or otherprocessed foods. In these instances, different upstream processing orhandling equipment is used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating the components of anextruder/dryer system in accordance with the invention used for theproduction of food or feed products;

FIG. 2 is a schematic view in vertical section of a product dryer inaccordance with the invention, shown during initial loading of the dryerwith wet product;

FIG. 3 is a graph illustrating the operation of the dryer of FIG. 2during initial loading thereof with wet product, depicting the buildupof SP and PV values, but without operation of a control variable;

FIG. 4 is a schematic view similar to that of FIG. 2, but illustratingthe dryer after full loading thereof with product;

FIG. 5 is a graph similar to FIG. 3 illustrating the operation of thedryer of FIG. 4 after full loading of the dryer, depicting the furtherbuildup of SP and PV values, but without operation of a controlvariable; and

FIG. 6 is a graph similar to FIG. 5 illustrating the further operationof the dryer of FIG. 4, depicting the calculated SP and PV valueswithout operation of a control variable, and initiation of the operationof a control variable CV, serving to drive PV towards SP until PV is atleast approximately equal to SP.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following Example illustrates an implementation of the presentinvention in the context of a horizontal, fuel-fired convection dryer.It should be understood, however, that this Example is provided by wayof illustration only, and nothing therein should be taken as alimitation on the overall scope of the invention.

EXAMPLE

The following hypothetical, computer-generated example illustrates animplementation of the present invention during the operation of anextrusion system 10 for the production of human foods or animal feeds.The system 10 broadly includes extruder 12 and a single-pass, fuel-firedconvection dryer 14. The extruder is itself conventional, and isoperable to produce a continuous stream of wet, cooked product 16, whichis processed in dryer 14 to yield a dry product 18. A control assembly20 is provided for the dryer 14 in order to determine, on a real-timebasis, the net rate of water leaving the wet product 16 during passagethereof through the dryer.

The control assembly 20 includes an exhaust fan 22 coupled with dryer14, and a wet bulb/dry bulb sensor 24 designed to measure the wet anddry bulb temperatures of the output of fan 22. Additionally, the fan 22is equipped with an rpm speed sensor 26 and a fan motor power sensor 28.The dryer 14 is further equipped with a combustion air blower 30, whichdelivers air to a fuel-fired air heater 32, so that the heated air isdelivered to the interior of dryer 14. An ambient air wet bulb/dry bulbsensor 34 is provided to determine the wet and dry bulb temperature ofthe input air, including the air fed to blower 30, and the make-up air36 passing into the dryer 14 as air is exhausted via fan 22. Atemperature sensor 37 is also operatively situated in dryer 14 forsensing the internal temperature thereof.

A programmable logic controller (PLC) 38 controls the operation ofassembly 20 using inputs from the wet bulb/dry bulb sensors 24, 34, thefan speed and power sensors 26, 28, and the controller for extruder 12.The PLC 38 output controls the operation of heater 32 by adjusting theflow of fuel thereto. Additionally, a display 40 is operably coupledwith PLC 38 so that the net rate of water leaving the wet product 16,and other control information, may be visually displayed to the operatorof system 10.

The dryer 14 is schematically illustrated in FIGS. 2 and 4, and includesan elongated dryer cabinet or housing 42 having an inlet 44 for wetproduct 16, and an outlet 46 for delivery of dry product 18 to atake-away conveyor 48 or similar device. Internally, the dryer 14includes a shiftable conveyor 50 presenting upper and lower runs 52, 54,and which is conventionally powered in order to continuously move thewet product 16 along the length of the dryer between inlet 44 and outlet46. The flow of wet product 16 is directed onto the upper run 52 ofconveyor 50. This incoming flow of wet product 16 has a determined WetRate, which is the total amount of product (both solids and native andadded water) making up the wet product as it issues from the extruder12. Additionally, the % H2O of the wet product 16 is known, i.e., thetotal % moisture of the wet product, based upon the total weight of thewet product taken as 100% by weight. This data is determined and storedwithin the extruder controller.

As illustrated, the dryer 14 also includes a combustion air blower 30and an associated heater unit 32, typically in the form of an open-flamefirebox. Further, the dryer has exhaust fan 22, which exhaustsmoisture-laden exit air from the cabinet 42 during operation. Make-upair 36 passes through the inlet 44 or any other suitable location. Aseries of recirculation fans 56, 58 recirculate the hot air within thedryer cabinet 42 during the course of the drying process. The exhaustfan 22 has known operating parameters or ratings, which are typicallyderived from the fan manufacturer, or can be independently determined.The fan 22 in this example has an operating volume of 16,000 ft³/minwhen operating under the following conditions: a rotational speed of1201 rpm, a pressure (wg) of 12 (FSP), power of 44.8 BHP, and airdensity factor of 0.0598 lb/ft³. The reference data was taken from a fansituated at an altitude of 1050 ft above sea level.

The dryer control process is implemented in PLC 38 by use of a PID loop.The PID loop requires a SP (set point), a PV (process variable), and ityields a CV (control variable). Successive calculations of SP and PV arefed into a PID loop to control the average net rate of water leaving theproduct 16 as it passes through dryer 14. The SP is the desired rate ofwater removal from the product 16 to achieve the desired moisture levelin the dry product 18. The PV is the net rate of water leaving theproduct 16 during passage through dryer 14. The CV is the temperaturewithin dryer 12, which is controlled by adjusting the fuel input toheater unit 32.

In practice, the calculations are performed essentially continuously,typically every 20 milliseconds. However, given the relatively slowresponse time of dryer 14, it is not necessary that each of thesecalculations be used in the control algorithm. Rather, the conveyor 50is divided into a series of N data elements, here 100 of such elements.The data interval is based upon the rate of travel and length of theconveyor 50. Calculations are fed to the data series at equal intervalsof time to represent each data element. Accordingly, each data elementrepresents the desired rate of water removal from the product 16 over acorresponding 1/100 of the length of conveyor 50. As each data elementis calculated by the PLC 38, it is stored in memory and all 100 of thedata elements, including those containing zero during dryer start-up,are averaged to obtain the average desired rate of water removal fromthe dryer 14. Each of the data elements is successively updated, and allthe stored data elements are averaged continuously. This provides amoving average of the desired rate of water removal from the product 16passing through the dryer 14.

The PID loop is initially tuned, which involves the process of selectingthe values of proportional, integral, and derivative gains of thecontroller to achieve the desired dryer performance. The selected valuesare chosen by considering a number of factors, including the type andsize of the dryer, the anticipated drying rate, and the type of productto be processed. Such tuning is well within the skill of the art.

SP, PV, and CV are calculated and used in PLC 38 as follows.

Calculation of the Set Point SP

In order to calculate SP, the PLC 38 needs the following information:

-   -   Wet Rate (WR)=the total rate of material (solids plus water,        both native and added) delivered to the dryer 14 as it issues        from the extruder 12, in kg/hr;    -   % H2O=the total moisture content of the wet product 16 issuing        from the extruder 12, in %;    -   Dry Rate (DR)=the total rate of bone dry material delivered to        the dryer 14 as a part of the wet product 16 issuing from the        extruder 12, in kg/hr;    -   Initial Water Rate (IWR)=the rate of water delivered to the        dryer 14 as a part of the wet product 16 issuing from the        extruder 12, in kg/hr;    -   Final Water Rate (FWR)=is the desired rate of water delivered        from the dryer 14 as a part of the dry product 18, in kg/hr;    -   Actual Water Rate (AWR)=the actual rate of water delivered from        the dryer 14 as a part of the dry product 18, in kg/hr;    -   Target Dry Moisture Content (TargetDMC)=is the desired moisture        content of the dry product 18, in %.        The WR, % H2O, and AWR are values derived from the operation of        extruder 12 and are delivered to PLC 38 as illustrated in        FIG. 1. The TargetDMC is a preselected value for the dry product        18. The remaining values are calculated as follows.        IWR(kg/hr)=WR(kg/hr)*% H2O(%)/100;        DR(kg/hr)=WR(kg/hr)−IWR(kg/hr);        FWR(kg/hr)=DR(kg/hr)/(1−TargetDMC(%)/100)−DR(kg/hr).

The final SP calculation is:SP(kg/hr)=IWR(kg/hr)−FWR(kg/hr).

Calculation of the Process Variable PV

PV=net rate of water removed from the wet product 16 during passagethrough dryer 14, in lb/hr, and is equal to (A) the rate of waterleaving the dryer 14 as a part of the output of exhaust fan 18, minus(B) the rate of water entering the dryer 14 as a part of the output ofcombustion air blower 30, and minus (C) the rate of water entering thedryer 14 as a part of the make-up air. Therefore, in order to calculatePV, the above three different values A, B, and C must be determined.

Determination of A=the Rate of Water Leaving the Dryer 14 as Part of theOutput of Exhaust Fan 22

This calculation includes the determination of the volumetric rate offlow of moisture-laden air from the exhaust fan 22, typically as CFM orf³/min. This determination is preferably carried out using anappropriate Fan Law, which in turn requires sensing of appropriate fanparameters. Such Fan Laws are described in Fan Engineering, 6th Edition(1961), edited by Robert Jorgensen, pp. 226-227. This reference teachesa number of ways of calculating fan CFMs, using different fanparameters. For example, Fan Law 10b calculates exhaust fan CFM usingsensed fan horsepower and fan rpm; Fan Law 7b calculates CFM using fanair pressure (which may be static, velocity, or total air pressure; asused herein, “fan pressure” refers to any of the foregoing) and fan rpm;and Fan Law 9c calculates CFM using fan motor power and fan pressure. Inprinciple, any of the CFM Fan Law equations could be employed, but froma practical point of view, 10b, 7b, and 9c are the most useful.Therefore, in preferred forms, two operational parameters of the fan areemployed, selected from the group consisting of: (1) fan motor power andfan rotational speed; (2) fan pressure and fan rotational speed; or (3)fan motor power and fan pressure.

For ease of reference, the preferred Fan Law equations are set forthbelow:CFM_(a)=CFM_(b)×(HP_(a)/HP_(b))^(3/5)×(RPM_(b)/RPM_(a))^(4/5)×(δ_(b)/δ_(a))^(3/5)  10b:CFM_(a)=CFM_(b)×(PRESS_(a)/PRESS_(b))^(3/2)×(RPM_(b)/RPM_(a))²×(δ_(b)/δ_(a))^(3/2)  7b:CFM_(a)=CFM_(b)×(HP_(a)/HP_(b))¹×(PRESS_(b)/PRESS_(a))¹×(1)  9c:where: CFM_(a) is the actual volumetric air flow of exhaust fan output,in f³/min; CFM_(b) is the fan manufacturer reference volumetric air flowof the exhaust fan output, in f³/min; HP_(a) is the actual horsepowerutilized by the fan motor, in BHP; HP_(b) is the fan manufacturerreference horsepower utilized by the fan motor, in BHP; RPM_(a) is theactual rotational speed of the exhaust fan, in rpm; RPM_(b) is the fanmanufacturer reference rotational speed of the exhaust fan, in rpm;δ_(a) is the actual air density of the exhaust fan output, in lbs/f³;δ_(b) is the fan manufacturer reference air density of the exhaust fanoutput, in lbs/f³; PRESS_(a) is the actual fan pressure of the exhaustfan; and PRESS_(b) is the fan manufacturer reference pressure of theexhaust fan.

In order to determine the value A using Fan Law equation 10b, the PLCneeds the following information:

-   -   mfT(R)=dry bulb temperature of the exhaust fan output in        Rankine;    -   mfTS(R)=wet bulb temperature of the exhaust fan output in        Rankine;    -   gfExtnAmbAlt=Dryer elevation (ft);    -   mfW=Absolute Humidity=lb water vapor/lb dry air in the exhaust        fan output;    -   mfWS=Saturated Absolute Humidity in the exhaust fan output=lb        water vapor/lb dry air in the exhaust fan output;    -   mfAP=atmospheric pressure of the exhaust fan output, in lb/in2;    -   mfPS=Saturation Pressure of the exhaust fan output, in lb/in2;    -   mfV=specific volume of the exhaust fan output, in ft³/lb;    -   CFMa=actual air flow of the exhaust fan output, in ft³/min;    -   CFMb=fan manufacturer-reference air flow of the exhaust fan        output, in ft³/min;    -   HPa=actual horsepower of the fan motor, in BHP;    -   HPb=fan manufacturer reference horsepower of the fan motor, in        BHP;    -   RPMa=actual rotational speed of the exhaust fan, in rpm;    -   RPMb=fan manufacturer reference rotational speed of the exhaust        fan, in rpm;    -   AirDensitya=actual air density of the exhaust fan output;    -   AirDensityb=fan manufacturer reference air density of the        exhaust fan output.

The mfT(R) and the mfTS(R) values are derived from the sensor 24. ThegfExtnAmbAlt is the system elevation. The HPb, RPMb, and AirDensityb areprovided by the fan manufacturer. The remaining values are calculated asfollows.mfPS=e(((14.61*mfTS(R))−8208.44)/(mfTS(R)72.60))mfWS=(0.6244*mfPS(lb/in²))/(mfAP(lb/in²)−mfPS(lb/in²))mfW=mfWS−((0.26*(mfT(R)−mfTS(R)))/(1359.0−(0.576*mfTS(R))))mfAP(lb/in²)=14.696(lb/in²)*(0.01367*(gfExtnAmbAlt(ft)/1000(ft/inHg))^(A)2−1.0744*(gfExtnAmbAlt(ft)/1000(ft/inHg))+29.92(inHg))/29.92(inHg)mfV(ft³/lb)=(mfT(R)/mfAP(lb/in²)*(0.3779(ft³/R*in²)+0.5909(ft³/R*in²)*mfW(lb/lb))AirDensitya(lb/ft³)=(1+mfWS(lb/lb))/mfV(ft³/lb)

Using Fan Law 10b from the previously mentioned Fan Engineeringhandbook:CFMa=CFMb*(HPa/HPb)^(⅗)*(RPMb/RPMa)^(⅘)*(AirDensityb/AirDensitya)^(⅗).

The value A is calculated as follows:A=the rate of water leaving the dryer 14 inlb/hr=CFMa(ft³/min)*AirDensitya(lb/ft³)*mfW*(lb/lb)*60(min/hr).

Determination of B=the Rate of Water Entering Dryer 14 as a Part of theCombustion Air of Blower 30

The combustion blower 30 has a fixed fan speed with a known CFM output.Therefore, the value B is calculated as follows:B=the rate of water entering dryer 14 as a part of the combustion air ofblower 30 in lb/hr=CFM of combustion blower 30(ft³/min)*ambientAirDensitya(lb/ft³)*ambient mfW(lb/lb)*60(min/hr).

Determination of C=the Rate of Water Entering the Dryer 14 as a Part ofthe Make-Up Air

The CFM of Dryer Make-up Air (ft³/min)=CFMa (ft³/min)−CFM of combustionblower 30 (ft³/min). The value C is calculated as follows:C=the rate of water entering dryer 14 as part of the make-up air inlb/hr=CFM of Dryer Make-up Air(ft³/min)*ambientAirDensitya(lb/ft³)*ambient mfW(lb/lb)*60(min/hr).

The final calculation of PV is:PV=A−B−C

Determination of the Control Variable CV

CV is used to control PV. In this implementation, CV=Dryer temperature.CV=Dryer Temperature(° F.)

FIG. 2 illustrates the system 10 during start-up as product is beingissued from extruder 12 onto conveyor run 52, and FIG. 3 depicts thedisplayed output from dryer control assembly 20. It is to be understoodthat the dryer 14 is typically preheated to a selected temperaturebefore actual drying operations commence. Accordingly, during theinitial fill stage, the heater 32 will not operate, because drying iseffected owing to the preheat of the dryer. However, as the processproceeds, the PV and SP lines moves upwardly and the heater 32 begins tooperate. Also as shown in FIG. 3, the PV curve is initially above the SPcurve, meaning that more water is being removed than is needed.

FIGS. 4 and 5 illustrate the system 10 once the dryer conveyor 50 isfully loaded. At this point, the extruder 12 is running at a constantrate and the PV and SP lines are becoming straight. Again, however, thedryer is in an “overdry” condition. FIG. 6 illustrates further progressof the dryer operations, again initially in an over dry condition.However, towards the center of FIG. 6, it will be observed that the CVbegins to alter the operation of the dryer 14, causing the PV plot toapproach the SP plot. Towards the right-hand end of FIG. 6, it will beseen that the operation of the CV has brought the PV and SP plots intoessentially full convergence, meaning that appropriately dried productsare now being produced. The control assembly 20 then will maintain thisoperational mode until some sort of upset occurs. For example, if theextruder 12 produces a wetter product, such information will be sent tocontroller 38 and the heater 32 will operate to provide additional heatto dry the wetter product. At the same time the successive calculationof SP and PV being fed into a PID loop govern the operation of theheater 32, as previously explained.

While the foregoing Example describes the implementation of the presentinvention in the context of a horizontal, single-pass, fuel-fired dryer14 having an open-flame heater or firebox 32, with a single combustionair blower 30, a single exhaust fan 22, and a single temperature sensor37, the invention is not so limited. For example, multiple-passhorizontal dryers could also be used having two or more verticallystacked conveyors. In such a case, the multiple conveyors would each bedivided into N data elements (such as 100 data elements per conveyor)and the total number of data elements would be stored in memory, toobtain the average net rate of water removal from the dryer.

Further, the dryer could be steam-fired where a bank of steam-fed finnedcoils is used in lieu of the heater or firebox 32, and a plurality ofrecirculation fans, such as the fans 56, 58, circulate ambient-deriveddrying air through the steam-heated coils to heat the drying air fordrying of the initially wet product. Likewise, depending upon the sizeof the dryer, a plurality of combustion air blowers and/or exhaust fanscan be used. Normally, in such a case, each of the exhaust fans would beequipped with one or more fan parameter sensors (e.g., rpm and fan motorpower), multiple temperature sensors 37 would be used, and the PLC wouldleverage the resultant data from the sensors to give SP and PV valuesduring the course of product drying.

The invention can also be used with vertical, multiple-deck dryers orcoolers. In the latter case, there would of course be no heated dryingair, but instead ambient air would be circulated through the cooler tobring down the temperature of the product and remove moisture therefrom.In essence, the invention may be used with any product moisture-removalapparatus, so long as the apparatus makes use of an exhaust fan with oneor more sensors for determining at least one fan operational parameter.

While the foregoing example sets forth a series of calculations todetermine the average net rate of water removal from the incoming wetproduct in real time, it will be appreciated that other calculationscould be used towards the same end. Although the use of a PLC with a PIDcontroller is preferred, it will be appreciated that other types ofdigital controllers may be employed, e.g., a personal computer.Similarly, although the invention has been exemplified through the useof hardware inputs and outputs between the various sensors and the PLC,standard wireless communication protocols (e.g., Ethernet IP, ModbusTCP/IP) could also be utilized. Although the use of an extruder 12 ispreferred, it should be understood that any suitable upstream processingunit (e.g., a pellet mill) could be employed, so long as it is capableof delivering a stream of wet product to be dried, with known Wet Rateand % H2O values.

We claim:
 1. A method of controlling the operation of apparatus operableto remove water from initially wet product passing through the apparatusby contacting the wet product with input air to create dried product andmoisture-laden exit air, said method comprising the steps of employingan exhaust fan to exhaust said exit air from the apparatus, determiningthe volumetric flow rate of said exit air from said exhaust fan, andusing said volumetric flow rate to determine the net rate of waterremoval from said wet product during operation of said apparatus, saidstep of determining the net rate of water removal from said wet productincluding the steps of: (1) determining the rate of water leaving theapparatus as a part of the exit air from said exhaust fan; (2)subtracting from rate (1) the rate of water entering the apparatus as apart of the input air, and as a part of said initially wet product toobtain the net rate of water removal from said wet product, andadjusting at least one control parameter of the apparatus in order toalter the rate of water removal from the wet product in response to thedetermined net rate of water removal therefrom, said control parameterselected from the group consisting of the temperature of said input air,the speed of said wet product passing through the apparatus, the contacttime between the wet product passing through the apparatus and saidinput air, and combinations thereof.
 2. The method of claim 1, includingthe step of heating a portion of said input air.
 3. The method of claim2, including the step of calculating said volumetric flow rate usingsaid fan parameter in a Fan Law equation.
 4. The method of claim 1,including the step of determining at least one parameter of said exhaustfan, said fan parameter selected from the group consisting of fan motorpower, fan rotational speed, and fan pressure, and combinations thereof,and using said at least one parameter to determine said volumetric flowrate.
 5. The method of claim 4, including the step of determining twooperational parameters of said exhaust fan selected from the groupconsisting of: (1) fan motor power and fan rotational speed; (2) fanpressure and fan rotational speed; or (3) fan motor power and fanpressure.
 6. The method of claim 5, including the steps of: (a)determining the wet bulb temperature and dry bulb temperature of saidexit air; (b) determining the rotational speed of said exhaust fan; (c)determining the motor power of said exhaust fan; (d) determining the wetbulb temperature and dry bulb temperature of said input air; and (e)determining said net rate of water removal from said wet product in realtime as the wet product passes through said drying chamber, using thedetermined values (a)-(d), inclusive.
 7. The method of claim 1,including the steps of determining a set point rate SP which is thedesired net rate of water removal from the wet product during passagethereof through the apparatus, determining a process variable PV whichis the actual net rate of water exiting the wet product during passagethrough the apparatus, and controlling the operation of the apparatususing said SP and said PV, to cause said PV to approach said SP.
 8. Themethod of claim 7, including the steps of successively and periodicallydetermining said SP and said PV, and controlling the operation of theapparatus using said successive SP and PV determinations.
 9. The methodof claim 8, including the step of using a PLC controller with a PIDcontroller to carry out said successive determinations of said SP andsaid PV.
 10. The method of claim 9, said PLC controller operable, ineach of said successive determinations, to calculate said SP, said PV,the difference between said SP and said PV, and a control variable CV.11. The method of claim 7, including the step of determining said SP asthe initial water rate (IWR) minus the final water rate (FWR), where IWRis the rate of water delivered to the apparatus as a part of said wetproduct input, and FWR is the desired rate of water exiting from theapparatus as a part of said dried product.
 12. The method of claim 7,including the step of determining said PV as the rate of water leavingsaid apparatus as a part of the output of said exhaust fan minus therate of water entering said apparatus as a part of said input air. 13.The method of claim 1, including the step of determining the net rate ofwater removal from said wet product in real time as the product passesthrough the apparatus.
 14. The method of claim 13, said net rate ofwater removal being an average net rate of water removal.
 15. A methodof controlling the operation of apparatus operable to remove water frominitially wet product passing through the apparatus by contacting thewet product with input air to create dried product and moisture-ladenexit air, said method comprising the steps of employing an exhaust fanto exhaust said exit air from the apparatus, and determining the netrate of water removal from said wet product in real time duringoperation of said apparatus, said step of determining the net rate ofwater removal from said wet product including the steps of: (1)determining the rate of water leaving the apparatus as a part of theexit air from said exhaust fan; (2) subtracting from rate (1) the rateof water entering the apparatus as a part of the input air, and as apart of said initially wet product to obtain the net rate of waterremoval from said wet product, and adjusting at least one controlparameter of the apparatus in order to alter the rate of water removalfrom the wet product in response to the determined net rate of waterremoval therefrom, said control parameter selected from the groupconsisting of the temperature of said input air, the speed of said wetproduct passing through the apparatus, the contact time between the wetproduct passing through the apparatus and said input air, andcombinations thereof.
 16. The method of claim 15, including the step ofheating a portion of said input air.
 17. The method of claim 15,including the step of determining at least one fan parameter of saidexhaust fan selected from the group consisting of fan motor power, fanrotational speed, and fan pressure, and combinations thereof, and usingsaid at least one parameter to determine said volumetric flow rate. 18.The method of claim 17, including the step of determining twooperational parameters of said exhaust fan selected from the groupconsisting of: (1) fan motor power and fan rotational speed; (2) fanpressure and fan rotational speed; or (3) fan motor power and fanpressure.
 19. The method of claim 18, including the steps of: (a)determining the wet bulb temperature and dry bulb temperature of saidexit air; (b) determining the rotational speed of said exhaust fan; (c)determining the motor power of said exhaust fan; (d) determining the wetbulb temperature and dry bulb temperature of said input air; and (e)determining said net rate of water removal from said wet product in realtime as the wet product passes through said drying chamber, using thedetermined values (a)-(d), inclusive.